Modest volcanic SO2 emissions from the Indonesian archipelago

Indonesia hosts the largest number of active volcanoes, several of which are renowned for climate-changing historical eruptions. This pedigree might suggest a substantial fraction of global volcanic sulfur emissions from Indonesia and are intrinsically driven by sulfur-rich magmas. However, a paucity of observations has hampered evaluation of these points—many volcanoes have hitherto not been subject to emissions measurements. Here we report new gas measurements from Indonesian volcanoes. The combined SO2 output amounts to 1.15 ± 0.48 Tg/yr. We estimate an additional time-averaged SO2 yield of 0.12-0.54 Tg/yr for explosive eruptions, indicating a total SO2 inventory of 1.27-1.69 Tg/yr for Indonesian. This is comparatively modest—individual volcanoes such as Etna have sustained higher fluxes. To understand this paradox, we compare the geodynamic, petrologic, magma dynamical and shallow magmatic-hydrothermal processes that influence the sulfur transfer to the atmosphere. Results reinforce the idea that sulfur-rich eruptions reflect long-term accumulation of volatiles in the reservoirs.

W hile not the most abundant species in volcanic gases, sulfur dioxide (SO 2 ) is the easiest to measure remotely with the aim of deriving a flux. This owes principally to its absorption of ultraviolet (UV) light, enabling daytime spectroscopic measurements from the ground, air and space 1 . Measurements of SO 2 flux are a cornerstone of volcano monitoring and contribute to the understanding volcanic degassing. They permit the calculation of fluxes of other volcanic gas species (X) from measurements of their ratios to sulfur dioxide (X/SO 2 ) and underpin global inventories of volcanic gas emissions to the atmosphere. In this respect, SO 2 is particularly important given its roles in atmospheric chemistry and radiation 2 .
Advances in satellite remote sensing of SO 2 in both ultraviolet and infrared wavebands are adding to our knowledge 3,4 and the proliferation of compact UV spectrometers and cameras [5][6][7] , is enabling measurements at less accessible volcanoes. However, the compilation of global inventories still faces numerous challenges, including temporal and spatial data gaps, measurement uncertainties, the presence of multiple sulfur species (including S, H 2 S and H 2 SO 4 ) in volcanic emissions, and the challenges of processing large volumes of data.
One notable lacuna in SO 2 inventories is the Indonesian archipelago. According to Siebert et al. 8 , there are 78 historically active volcanoes in Indonesia, i.e., those with at least one historically-recorded eruption. But such a definition finds its limit in Indonesia where the documentary record is incomplete and traditional knowledge lost or not fully integrated into scientific records. A more complete inventory of the Indonesian volcanoes can be found in the "Badan Geologi" database 9 which lists 126 active volcanoes (including six submarine edifices). They are subdivided into 77 type-A volcanoes, which have experienced at least one increase in magmatic and/or phreatic activity since 1600; 29 type-B volcanoes with solfataric and/or fumarolic manifestations but no eruption since 1600; and 20 type-C, which are solfataras and/or fumarole fields lacking a defined volcanic edifice ( Fig. 1 and Table 2). These volcanoes fall within four distinct arcs: Sunda, Banda, Sangihe, and Halmahara ( Fig. 1).
In the first global compilations of the volcanic SO 2 budget, the Indonesian contribution was unspecified 10 . Subsequently, over four decades, new observations have furnished estimates of annual SO 2 inventories for Indonesia (Table 1). These have varied considerably, beginning with the work of Le Guern 11 , who estimated 0.07 Tg /yr (representing just 0.15% of the global volcanic SO 2 budget) compared with a more recent estimate of Carn et al. 3 of 2.2 Tg SO 2 /yr (representing 9.5% of the global volcanic budget). Note that these and intervening works have also reported disparate figures for the global total (Table 1), which is unsurprising given the different datasets (and their timespans) and methods employed. Despite developments in SO 2 sensing, hitherto only a fraction of the more than 100 Indonesian volcanoes classified as active has been subject to SO 2 flux measurement campaigns.
Here we present a new inventory of volcanic SO 2 emissions for Indonesia based on portable ground-based remote sensing instruments and systematic program of fieldwork observations. We focus our efforts on the subaerial type-A volcanoes with passive degassing, which we consider based on field observation to be the main volcanic degassing sources in Indonesia (Table 2 and Fig. 1). We use the term 'passive' to refer to the style of gas emission so as to distinguish it from larger, sporadic explosive emissions, though the term can encompass a wide range of sources from magmatic to fumarolic. We evaluate the factors influencing the variations identified between volcanoes and between sub-regions of Indonesia and consider the total SO 2 emission rate for the archipelago in the global context.

Results
SO 2 emission budget. Of the 73 aerial type-A volcanoes across Indonesia, we conducted measurements at 47 ( Fig. 1 and Table 3), including 12 that exhibit negligible SO 2 release. Of the remaining 26 volcanoes that were not visited, 20 are either inactive or exhibit negligible SO 2 emission, according to local observatory reports and available data and images (https://vsi. esdm.go.id/). There were six volcanoes known for persistent degassing that we did not visit: Banda Api, Serua, Batu Tara, Sangeang Api, Rinjani, and Arjuno Welirang (Fig. 2). However, satellite observations provide some constraints for Batu Tara, Rinjani and Sangeang Api 3 .
The Sunda arc makes the largest contribution, with a collective daily output of 1313 ± 539 Mg (Fig. 2) (Table 3). Two other volcanoes of the Banda arc, including Serua, and Banda Api, were not visited and therefore their SO 2 emissions remain unknown. However, based on information from the local observatories, degassing strength of Banda Api is comparable to that of Wurlali, and exceeds that of Serua. The lack of measurements is thus unlikely to bias significantly our arc-scale flux estimate. The SO 2 degassing associated with low temperature solftaras and fumaroles from the remaining eight type A volcanoes of this arc is negligible ( Table 3).
The Sangihe arc hosts three volcanoes with relatively strong SO 2 emission rates, including Soputan (376 ± 100 Mg/d), Lokon (117 ± 10 Mg/d) and Karangetang (120 ± 55 Mg/d). Awu the northernmost volcano of the arc emits 13 ± 5 Mg SO 2 /d. The SO 2 contribution from the five remaining volcanoes is negligible (Table 4). Hence, with a total SO 2 degassing budget of 626 ± 170 Mg per day, the Sangihe arc constitutes a notable arcscale volcanic degassing source to the atmosphere. Note, however, that Soputan clearly stands out as the strongest source, representing 60% of the total for this arc.
Finally, the SO 2 emission rate from the Halmahera arc amounts to 897 ± 437 Mg/d with more than 90% of this flux accounted for by Dukono which emits 819 ± 394 Mg/d. The other volcanoes are low to moderate sources, with 59 ± 32 Mg/d from Ibu, 16 ± 10 Mg/d from Gamalama and 3.4 ± 1.0 Mg/d from Gamkonora. On Kie Besi, only a small fumarole is present and its SO 2 emission is considered negligible.
Based on the above results, and as summarized in Table 3, the total daily SO 2 emission passively released into the atmosphere from the entire Indonesian archipelago is 3200 ± 1300 Mg/d, equivalent to 1.15 ± 0.48 Tg SO 2 yr −1 (Table 3). We emphasise that this figure is representative of the periods of observations and must be viewed cautiously but we believe it gives a useful guide to the scale of emissions at the scale of the entire archipelago.
Principal point sources. Our ranking of Indonesian volcanic sources of passive degassing is shown in Table 3. Dukono (Halmahera arc) is the strongest, representing more than a quarter (26%) of the total. Soputan (Sangihe arc), Sinabung and Kawah Ijen (Sunda arc) are also notable, representing, 12%, 9% and 8% of the total, respectively. These four volcanoes alone constitute around half of the total inventory. They are sustained by different magma compositions, i.e., basaltic andesite to andesite on Sinabung 13  between 50 and 100 Mg/d that together represent 11% of the budget, seven with SO 2 emission rates between 10 and 50 Mg/d, representing 5% of the budget, and finally 14 volcanoes whose SO 2 degassing is below 10 Mg/d.

Arc scale variations.
Our new SO 2 inventory reveals substantial variations in SO 2 output between the arcs of the Indonesian archipelago. The 3000-km-long Sunda arc, with 37 type A volcanoes, is the largest SO 2 source at 0.48 ± 0.20 Tg/yr representing 41% of the Indonesian total. The 2000-km-long Banda arc, in contrast, contributes just 0.12 ± 0.06 Tg/yr representing only 10% of the total, despite hosting 24 Type A volcanoes. The 600-kmlong Sangihe and 500-km-long Halmahera arcs are stronger sources (0.23 ± 0.06 Tg and 0.33 ± 0.16 Tg SO 2 yr −1 , respectively) despite their shorter extents and comparatively few volcanoes (eleven and five, respectively). It is thus plausible that the geodynamic contexts play a key role in the SO 2 emission budget of Indonesia; in terms of SO 2 emission rate per km of arc, the Halmahera arc is the strongest source with an output of 655 Mg SO 2 per km yr −1 followed by the Sangihe, Sunda and Banda arcs with 380, 160 and 60 Mg SO 2 per km yr −1 , respectively.
Passive and explosive degassing. Our total Indonesian SO 2 inventory of 1.15 ± 0.48 Tg/yr based on ground-based and airborne surveys of persistent volcanic degassing across the archipelago is half the estimated emission from the 20 Indonesian volcanoes reported in ref. 3 , derived from satellite remote sensing measurements. But this latter approach focused on a different  (Table 4 and Fig. 3). The Sunda arc dominates this list with 60 eruptions at 16 different volcanoes. Sinabung and Anak Krakatau were the most active with nine and eight events, respectively. Kerinci and Marapi were also notably active with, respectively, six and seven eruptions, whilst Merapi, Semeru, and Sangeang Api experienced four eruptions each. The Banda arc produced 13 eruptions at seven different volcanoes over the last decade. Batu Tara was the most active with four eruptions. For the Sangihe arc, 19 eruptions were reported at three different volcanoes, including Karangetang, the most active with ten eruptions, Soputan with six eruptions and Lokon with three eruptions. Eighteen eruptive events were recorded for the Halmahera arc. Dukono was the most active volcano with 11 eruptions, followed by Gamalama with six eruptions.
To estimate the SO 2 contribution from these explosive events, we first used a formulation 16 relating volcanic explosivity index (VEI) and SO 2 yield: We took VEI values reported in the Global Volcanism Program (https://volcano.si.edu/). This indicates a total eruptive SO 2 output over the 2010-2020 period of 5.99 ± 0.31 Tg with annual totals varying between 0.25 Tg and 1.03 Tg (Table 4, Fig. 3). The Sunda arc released 3.7 ± 0.3 Tg, representing 62% of the total, with the main contributions from Sinabung, Merapi, Kelut, Anak Krakatau, and Agung. For the Banda arc we estimate 0.40 ± 0.05 Tg SO 2 (0.04 Tg/yr) accounting for 7% of the budget, mostly contributed by Rokatenda, Lewotolo and Batu Tara. For the Sangihe arc, Soputan, Karangetang and Lokon volcanoes were the only sources with an estimated combined yield of 0.75 ± 0.05 Tg (0.07 Tg/yr) of SO 2 or 12% of the total. Lastly, the Halmahera arc released 1.12 ± 0.01 Tg (0.10 Tg/yr) of SO 2 through eruptions, representing 19% of the total. Dukono was the main contributor, accounting for 83% of the arc's output (Table 4).
We also analyzed available satellite data (http://SO2.gsfc.nasa. gov/) for the SO 2 mass over Indonesia between 2010 and 2020. Out of the 110 eruptions, 71 were captured by the satellite (64%) and the corresponding SO 2 mass amounts to a total of 1.31 ± 0.18 Tg, with a mean annual value of 0.12 ± 0.04 Tg. The Sunda and Halmahera arcs are the main contributors representing, respectively, 81% (1.07 ± 0.18 Tg) and 11% (0.14 ± 0.01 Tg) of the total SO 2 . The main contributors from the Sunda arc are Kelut (44%), owing to its 2014 eruption 17 , Merapi (14%), primarily related to its 2010 event 18 , and Sinabung (12%), which has experienced episodic dome growth and collapse since 2010 (ref. 19 ). For the Halmahera arc, 99% of the arc contribution is from Dukono, reflecting its continuous eruptive activity. For the Sangihe arc, the total SO 2 yield amounts to 0.03 ± 0.005 Tg, and Soputan, with its recurrent eruptive activity, is the main source, representing 67% of the arc contribution. Finally, the Banda arc contribution corresponds to 0.068 ± 0.019 Tg and is dominated by the 2020 eruption of Ili Lewotolo, representing 92% of the arc contribution. These figures obtained from satellite data are lower than those calculated from the VEIs (Fig. 3 and Table 4), except for Kelut and Ili Lewotolo, where the estimates converge. Despite the discrepancy, particularly in the lower SO 2 estimates, both approaches highlight the Sunda and Halmahera arcs as the main SO 2 contributors to explosive emissions. Combining the figures we derive for passive and explosive SO 2 degassing yields a total source between 1.27 and 1.69 Tg SO 2 /yr for the Indonesia archipelago. The lower and the higher range integrate respectively the mean annual figures from satellite data and from the VEIs (Table 4). About 10% to 30% of total SO 2 release was sustained by larger, sporadic explosive emissions. This Indonesia total budget corresponds to 3-7% of the global volcanic SO 2 emission budget based on estimates in ref. 3 and 20 (23-33 Tg/yr) and is comparable to the total emissions from Japan 21 ( Fig. 4), although much less when the degassing budget is normalized by arc length.

Discussion
This work constitutes the first near-comprehensive SO 2 emission survey across the Indonesian archipelago. We estimate a passive degassing flux of 1.15 Tg SO 2 /yr for Indonesia in the period of 2010-2019. This represents the cumulative emission from twenty volcanoes, the four strongest sources being Dukono, Soputan, Sinabung and Kawah Ijen, which together represent 54% of the total emission budget, while Slamet, Anak Krakatau, Bromo, Karangetang, and Lokon account for another 25% of the total. More modest sources include Batu Tara, Ili Lewotolo, Rinjani, Sangeang Api, Rokatenda, and Ibu, representing 14% of the total. Seven minor sources, Semeru, Slamet, Merapi, Gamalama,  Le-  wotobi Perempuan and Awu account for the remainder. We have also estimated explosive SO 2 emissions from Indonesia for the period 2010-2020 based on a simple scaling from reported VEI values and satellite records. The mean annual explosive-SO 2 obtained range between 0.12 and 0.54 Tg/yr, 63-81% of which is associated with the Sunda arc (0.10-0.34 Tg/yr), 5-7% (0.04-0.07 Tg/yr) the Banda arc, 3-13% (0.03-0.07 Tg/yr) the Sangihe arc and 11-18% (0.10-0.14 Tg/yr) the Halmahera arc. Combining the calculated passive and explosive SO 2 data suggests a total volcanic SO 2 yield for the Indonesian archipelago of 1.27-1.69 Tg/yr. We consider this a representative figure, acknowledging that it is based on very limited temporal sampling of the volcanoes in question.
While the overall SO 2 budget is unremarkable, the bulk of the emissions are into the free troposphere (Fig. 4D) likely to extend timescales of atmospheric processing and deposition of sulfur 27 . Previous studies have highlighted the contribution of sulfur deposition from volcanic plume to sulfur emissions from peat fires 28 .
Factors controlling sulfur output are numerous and include deep source characteristics and chemical processes occurring during magma storage and transfer through the crust. Hydrothermal scavenging and scrubbing of sulfur from magmatichydrothermal discharges is often invoked as a process for sulfur  29 . Acidic crater lakes, which are numerous in Indonesia, are perhaps the most obvious manifestation of such processes 30 . Substantial sulfur deposits are known to be sequestered by volcanic lake systems and, conceivably, variations in climatic conditions, notably rainfall, across the archipelago could play a role in volcanic emissions to the atmosphere.
The subaerial sulfur output will depend on initial gas composition, the flow path, gas-wall rock heat transfer, and the effective water to rock ratio, all parameters that are difficult to constrain and which vary greatly between volcanoes. However, were hydrothermal scavenging and scrubbing leading mechanisms, one would expect flux strength to correlate with the concentration of sulfur in fluids. Instead, for the few volcanic centres for which, in addition to SO 2 flux, there are constraints on H 2 O, CO 2 , H 2 S + SO 2 species in fumaroles, we find no correlation between the mass fraction of sulfur in the fluid, XS tot (=XSO 2 + XH 2 S) and SO 2 flux (Fig. 5A), which presumably is a function of both the initial volatile content of the magma and degassing conditions. Dukono's gas, in particular, does not have higher sulfur than other Indonesian fumaroles (the case of Krakatau needs to be confirmed by more measurements). This indicates that SO 2 flux need not reflect particular enrichment/depletion in sulfur of the emitted gas, and implicates the role of degassing vigour during the time interval considered (which can scale with conduit radius and presence of an open vent to the atmosphere). From this perspective, it is worth emphasising that to establish robust links between volcanic degassing and processes at depth, it requires comprehensive measurement of gas composition.
While hydrothermal sequestration of sulfur is likely to play a significant role in modulating subaerial emissions, we consider also whether arc scale differences in SO 2 emissions across the Indonesian archipelago might reflect geodynamic or source controls, as proposed for CO 2 in arc magmas worldwide 31 . The amount of SO 2 released per km of arc per year reveals the Halmahera arc as the strongest SO 2 source, followed by the Sangihe arc. The magmatic sources of these two arcs are sustained by the double subduction of the Molucca Sea plate that deepens to the west beneath the Sangihe arc, and to the east under the Halmahera arc 15 . Geochemistry of lavas sampled along these arcs indicates enriched magma sources in fluid-mobile elements and notable sediment contributions 32,33 , which may play a role in subaerial sulfur budgets. The steepening of the subducted slab, the downward force from the Philippine Sea plate, and the westward motion of the continental fragments along the Sorong fault could have promoted fluid fluxes into the mantle wedge along the Halmahera arc 15 . To a first approximation, this peculiar geodynamic context may explain elevated SO 2 fluxes at both Halmahera and Sangihe arcs simply because of enhanced magmatic activity.
In contrast, the Banda arc stretches 2000 km but exhibits a remarkably low SO 2 emission, the weakest in our inventory. The arc is also characterized by anomalously low 3 He/ 4 He ratios 34 reflecting the arc collision with the Australian continental block and subduction of continental material that ultimately supplies less sulfur to the mantle wedge, compared to subduction of oceanic plate 35 .
The Sunda arc is the largest SO 2 source, representing 43-48% of the total, however, its annual SO 2 emission per km of arc is modest compared with the Halmahera and Sangihe arcs, and with other arcs worldwide 20 . The magma source beneath the Sunda arc is sustained by subduction of the Indo-Australian plate. However, while deep sea drilling has revealed a 1400 m sediment column in front of Sumatra, 300 m in front of Java, and 500 m in front of Sumbawa 36 , less than 15% of these sediment columns is subducted 37 . The mass transfer along the Sunda arc is dominated by an active frontal accretionary prism that strongly limits sediment subduction. Each year only 2.6 × 10 7 m 3 of sediment is subducted beneath the Sunda arc, compared with the 1.8 × 10 8 m 3 /yr available, given the average subduction speed of 6.7 cm/yr. Furthermore, the sediment input from the Sunda arc is mostly trapped in the forearc basin and does not reach the trench. Given that subducted sediment can strongly contribute to the volcanic sulfur budget, it is possible that this active accretional prism plays a key role in modulating the SO 2 emission budget of the Sunda arc by limiting the mass transfer of sediment-derived sulfur into the mantle wedge. However, as shown in the next section, such a variability in sediment contribution is not evident in variable sulfur abundance in magmas sustaining Indonesian arc volcanism.
Arc volcanoes are typically supplied by reservoirs in the shallow crust, which are in turn fed by basaltic melts rising from the mantle wedge and carrying an imprint of slab volatiles. Because they are hot, these mafic magmas have a higher sulfur carrying capacity than cooler silicic magmas, hence any volcano erupting mafic magmas should generally be associated with stronger sulfur emissions (though of course arc-scale variations in mafic melt sulfur content are possible). Detailed petrological studies of Indonesian volcanoes remain scarce, and only a few have had their sulfur content characterized via analysis of melt inclusions (MI). Figure 5B draws on these studies and shows the highest sulfur contents measured in MI along the Indonesian arcs. Drawing rough relationships from these data, basaltic MI have 2000-3500 ppm S, andesitic MI 800-500 ppm S, and rhyodacitic 200-300 ppm S, with no obvious geographical trends along arc being apparent, assuming that primary sulfur contents in the melt inclusions are little affected by post entrapment processes.
The most evident feature is that Dukono, the strongest SO 2 source we identify, has basaltic melt inclusions with rather low S content (1000 ppm) relative to other, currently weaker SO 2 sources, such as Rinjani or Kawah Ijen, which have MI with sulfur in excess of 2000 ppm. Similarly, Soputan emits half the SO 2 of Dukono, yet has basaltic MI with much higher sulfur content (3500 ppm). This is illustrated in Fig. 5C, which shows that a broad positive correlation between SO 2 flux and the maximum sulfur content, S max , in MI. Only Dukono departs significantly from the trend indicated by other centres. This again argues for strong decoupling between the fertility of the immediate source of magma degassing (the crustal reservoir) and its ultimate surface manifestation. Such a behavior may simply reflect conduit dynamics, such as convection, which is strongly dependent on conduit radius [38][39][40] , and which can sustain strong SO 2 degassing of an otherwise comparatively sulfur-poor reservoir. Alternatively, a more sulfur-rich magma may give rise to a low sulfur output simply because of a low magma influx that cannot sustain conduit convection. A critical parameter is the conduit radius, R, since magma (and gas) flux scales with R 4 (ref. 40 ) such that small variations in conduit size can result in large fluctuation in sulfur flux.
The composition and temperature of the magma supplied to the conduit will also be important, owing to their influence on viscosity and rheology. Hotter and fluid material will promote not only higher rates of magma overturn in the conduit but also more efficient degassing of slowly diffusing species, such as sulfur, in silicate melts. From this perspective, systems lying above the dashed line toward Soputan, whose high SO 2 flux may be considered as directly related to its basaltic magma source 14 and high sulfur content in MI 41 (Fig. 5C), may reflect more pronounced scavenging by the aquifer/hydrothermal system overlying magma reservoirs, limiting sulfur emissions to the atmosphere. These local controls will be superimposed on any deeper source signatures and may even obliterate them, as exemplified here by Dukono.
The SO 2 emission budget of the Indonesian archipelago thus reflects a complex interplay between deep (geodynamic) factors that control primary magma compositions and their availability along the arc and superficial processes such as hydrothermal scavenging and conduit dynamics. Reservoirs regularly supplied by fresh hot magma may promote sustained and vigorous degassing via conduit convection, leading to strong SO 2 outputs even when sulfur complements in the melt are comparatively poor. In other words, the vigour of convection may largely compensate for, or even offset, any deep source deficiency in sulfur. The relatively low SO 2 output for the Indonesian archipelago documented here may appear in stark contrast with the record of explosive eruptions at several Indonesian volcanoes and their recognized global climate impacts (e.g. Tambora, Krakatau, Rinjani). These events, however, essentially reflect the long term accumulation of magmas and volatiles in closed crustal reservoirs, which cool and fractionate with little volatile loss, a process that differs from the persistent or passive degassing operating at openconduit systems such as those we document here. An in depth knowledge of the petrology of volcanic products, and a robust characterization of emanating fluids, are both required if sound connections between the plumbing system and degassing are to be established at any active volcano.

Methods
Passive ultraviolet spectrometers. We used two techniques, USB-controlled ultraviolet spectrometers and Differential Optical Absorption Spectroscopy (DOAS) 42 and ultraviolet cameras (UV-cam) 6 . The passive ultraviolet spectrometers were either carried beneath the plume on a moving platform 43 or located in a fixed position and attached to scanning optics 7 . Both approaches yield the SO 2 profile across the plume. The spectrometers used were the Ocean Optics USB2000 (280-400 nm, 0.5 nm FWHM resolution), USB4000 (290-440 nm, and 0.3 nm FWHM), and USB2000 + (290-440 nm and 0.5 nm). For traverse measurements, the spectrometer was connected via an optic fibre bundle to a vertically pointed telescope of 8 mrad FOV (Field Of View). The location of each recorded spectrum was obtained using a continuously recording GPS unit. The DOAS traverse setup requires no additional power supply since the spectrometer is powered by the laptop. We operated the equipment onboard a light aircraft, from a 4WD vehicle, and on foot.
For the scanning observations, we used a rotating window that accepts light from selected directions across the plume. The light that transits through the window is redirected to an embedded telescope by a 45°optical prism, then transmitted to the spectrometer via optical fibre. The rotating window was attached to a stepper motor controlled by the laptop via a microcontroller. The system was designed to perform a 180°scanning angle with a minimum step angle of 1.8°. The scanner required an external 12 V power supply. The scanning setup could be readily operated by one person. Spectra were acquired using Jscript executed by DOASIS software 44 . The script used in both traverse and scanning allowed optimization of the signal-to-noise ratio by automatically adjusting exposure time and numbers of co-added spectra 45 . This was particularly useful for scanning, given the change of light intensity with scan angle. Both traverse and stationary recording were carried out at distance varying between few tens of meters from the craters to around 5 km downwind, depending on the access difficulties, the plume size and the volcanic activity. SO 2 column amounts (ppm m) were retrieved using standard DOAS calibration and analysis procedures outlined in ref. 43 . Reference spectra included in the nonlinear fit were obtained by convolving high-resolution SO 2 and O 3 cross-sections with the instrument line shape. A Fraunhofer reference spectrum and ring spectrum, calculated in DOASIS, were also included in the fit. The optimum fitting windows were selected where they provided a near-random fit residual with minimum deviation. The total SO 2 column amount across the plume was then multiplied by the estimated plume speed to obtain the SO 2 flux. The plume velocities were measured mainly using videography and handheld anemometers, except in the case of airborne measurements where the plume speed was obtained by flying along and against the plume axis.
Ultraviolet cameras. The imaging setup consisted of two Apogee Alta U260 UV cameras. Each was coupled to a Pentax B2528-UV lens, with a focal length of 25 mm allowing a full angle FOV of around 24°. Immediately in front of each lens, a 10 nm (FWHM) bandpass filter was placed, one filter was centered at 310 nm (Asahi Spectra XBPA310) where SO 2 absorbs and the other at 330 nm (Asahi Spectra XBPA330) outside the SO 2 absorption region. Image acquisition and processing were achieved using Vulcamera 46 . For each pixel the optical depth (OD) was obtained according to the following equation: OD ¼ Àlnf½ðPA À DAÞ=ðCA À DAÞ=½PB À DB=ðCB À DBÞg where A and B represent the camera with the 310 nm and 330 nm filters respectively, and P, D, and C represent plume, dark and clear images. To correlate the OD values with the SO 2 slant column densities (SCDs), four calibration cells with known amounts of SO 2 (94, 189, 475, 982 ppm.m) were used. Calibration images were acquired at the beginning of measurements and repeated with long series of measurements. The UV-cam was generally positioned with a view perpendicular to the plume transport direction, at distance between <1 km and 6 km depending on plume size. Plume speeds were derived during data processing by following plume structures between two fixed lines perpendicular to plume transport direction 46 .
Uncertainties. Uncertainties in DOAS and UV-cam SO 2 flux measurements are discussed in many past works, including the following ref. 47 . The dominant error in the retrieved SO 2 column amount is induced by the variability of light intensity and the distance between the plume and instruments. With increasing distance, light that has not traveled through the plume may contribute significantly to the signal. This leads to light dilution of the plume signal that can easily cause more than 50% underestimate in SO 2 emission rate 48 . To reduce this effect in the UVcam measurement, we deployed the system during clear sky conditions, at distances <6 km, and performed calibration every hour during long series of measurements. UV-cam measurements were performed mainly in the late morning before the clouds started to formed, generally at 9-11 am.
For the DOAS measurement, we compensate for light intensity changes using an artificial constant dark, calculated from each recorded spectrum, in the 'UV blind' region (below 290 nm). Such corrections account for dark spectrum, offset and stray light. We estimate that the error in the column amount contributes~0.01 to the squared variation coefficient of the total flux. We also assumed that the plume and transport direction is homogeneous and in a straight line since it is difficult to rigorously assess in this work. We, therefore, performed flux calculations for direction ϕ, ϕ±3, and ϕ±6. The mean contribution to the square variation of the total flux is in the order of a thousandth. These errors are negligible in comparison to uncertainties in the plume speed that resulted from the complexity of wind field around volcanoes and frequent variations in both time and space. The plume transport speed relative error is conservatively assumed to be about 30-35%, which is towards the higher end of the range of past estimates 49 . These errors are applied to each traverse and profile then the mean value is calculated for each series of measurement with the corresponding standard deviation. The global estimates for the arc is the sum of the mean values.

Data availability
The data that support the plots within this article are available from the corresponding author upon reasonable request.